Currently, the spectrum allocated for mobile communications corresponds to the Ultra-High Frequencies (UHF) band, laying in the 300 MHz-3 GHz frequency range. This spectrum may in the future not be able to host all the traffic generated inside the mobile networks. Spectrum in the millimeter-wave band (MMB), a portion of the radiofrequency (RF) spectrum ranging from 3 GHz to 300 GHz, is at present underutilized.
The generation of ultra-stable RF signals from UHF to MMB is made possible by resorting to photonic technologies, as described in Seeds, A. J. et al., “Microwave Photonics”, JLT, vol. 24, Issue 12, p. 4628-4641, December 2006. The use of MMB may involve implementing a beamforming by means of arrayed antennas, so that the range covered by the system is maximized in the desired direction.
Phased array antennas (PAAs) allow steering of transmitted Radio Frequency (RF) beam without physically moving the antenna. Phased array antennas are used in an increasing number of applications such as multifunctional radars and communications.
It is known for PAAs to use electronic phase shifters at each antenna element to control the viewing angle of the array. For broadband signals, this approach suffers from the squint phenomenon, which causes different frequencies of the RF signal spectrum to aim at a different angle. As is also known, squint can be avoided if the phase shifters are substituted by true-time delays (TTDs).
A photonics approach has been proposed for realizing the TTD functionality in PAAs, utilizing the photonics capability of controllable delays with wide bandwidth, avoiding beam squint. Photonics also have with the advantages of low weight and Electro-Magnetic Interference (EMI) insensitivity. An optical tunable TTD has been demonstrated through optical path switching, as described in A. P. Goutzoulis, et al., Opt. Eng., v. 31, pp. 2312-2322, 1992. An optical tunable TTD with dispersive elements is described in K. Prince, et al., IEEE J. Lightwave Technol., v. 27, n. 22 (2009). Such photonics-based solutions require a generation of an RF signal, to which the TTD delay is subsequently added.